The most likely cause of this problem is that you've disabled the sending of read receipts within Outlook Express. Open Outlook Express and choose Options from the Tools menu. Select the Receipts tab. In the middle of the dialog you'll see three radio buttons relating to the default behavior for receipts. If 'Never Send a Read Receipt' is selected, we've found your problem. Select 'Always Send a Read Receipt' and you'll be good to go.
This was first published in May 2001